This invention relates to polyurethane foams having increased fire resistance, and is particularly concerned with the production of fire resistant polyurethane foams by incorporation of certain phosphorus compounds into the foams.
It is known in the prior art that the fire resistance of polyurethane foams can be increased by chemical incorporation of phosphorus compounds into the foams. Thus, for example, U.S. Pat. No. 3,986,990 discloses polymerizing polyalkylene glycol alkyl polyphosphites with or without additional polyols, with selected isocyanates, thereby producing polyurethane foams in which these polyphoshites are chemically incorporated into the polyurethane network. U.S. Pat. No. 3,989,652 discloses the preparation and subsequent incorporation of polyalkylene glycol alkyl or halo-alkyl polyphosphonates into the polyurethane polymer chain by chemical bonds.
In the above prior art, the attachment is chemical via a reaction between the isocyanate and an active hydrogen which is part of the phosphorus-containing molecule. This results in incorporation of the phosphorus atom into the main polymer backbone which leads to loss of commercially attractive properties, such as softness and resilience in flexible foams, and lack of friability in rigids. Susceptibility to hydrolysis is also increased for both rigid and flexible foams. These problems are so acute for flexible foams that present commercial practice is to introduce the phosphorus in a non-reactive form, e.g. as tris-(2-chloroethyl phosphate). This results in a small degree of plasticizing, but more importantly, the fire retardant is simply dissolved in the foam polymer, and in due course it diffuses out, and hence fire retardance is lost.
It is known to copolymerize two different monomer moieties resulting in the formation of two intertwined but chemically separate networks. Thus, Belgian Pat. No. 843,641 discloses simultaneous polymerization of a polyol and a polyisocyanate, together with polymerization of an ethylenically unsaturated monomer such as a substituted styrene, e.g. mono-chlorostyrene, to form intertwined networks of polyurethane and polymerized ethylenic monomer.